1. Field of the Invention
This invention relates to a process of preparing and making of photographic emulsions. Specifically, this invention relates to a particular method for the addition of a powdered material such as dyes to a photographic system. Examples of dyes include spectral sensitizing dyes, filter dyes and antihalation dyes. Also the present invention relates to emulsions containing such powders.
2. Discussion of the Prior Art
During the preparation of gelatino, silver halide emulsions, the addition of a powder such as a dye is useful such as for spectral sensitization, image quality improvement or antihalation. Illustratively in the case of sensitization, such property may take the form of so-called "chemical sensitization" wherein various ingredients are added to change the sensitometry thereof. Alternatively, it may also be necessary to alter the spectral response of the emulsions for one reason or another. If this is required, so-called "spectral sensitizing" dyes are added to this aqueous dispersion of gelatin and silver halide. As well known in the art spectral sensitizing dyes are not very soluble in either an aqueous or in a mixture of organic/aqueous solvents. Adding the dye as a solution therefore requires large volumes of material only a small fraction of which actually provide useful photographic properties and the rest adds volume which must be subsequently removed. Furthermore, it is well known in the art that many dyes began to decompose in solution resulting in decomposition products which may be detrimental to the photographic properties of the emulsion. This typically requires the establishment of a shelf life after which the dye solution is discarded thereby adding substantial cost to the overall operation.
Alternatives to adding the dye dissolved in a solution are described in the literature. Dispersion of the dye in hydrophilic colloid as exemplified in Owens, et al., U.S. Pat. Nos. 3,660,101 and 3,469,987; Ihama et al., U.S. Pat. No. 4,683,193. These methods require various combinations of heating cycles, precipitation, and high shear stirring all of which are time consuming and detrimental to the dye as mentioned above. The use of surfactants has also been taught, however, this method requires the addition of surfactants to the emulsion which may alter the photographic properties of the emulsions. Microencapsulation has been employed extensively in the art as exemplified in Keys, et al., U.S. Pat. No. 4,755,446 and Nelson, U.S. Pat. No. 4,798,741. Microencapsules are formed by various methods such as coacervation, interfacial polymerization, polymerization of one or more monomers in an oil, etc., with a typical result being a microcapsule with a size of 0.1 to 25 .mu.m. Extreme care must be taken with these techniques since the dye must be completely inert to the chemical reactions involved in microcapsule formation. Furthermore, during the formation of microcapsules, impurities such as solvent are typically occluded within the microcapsules and therefore are added to the emulsion with the microcapsules.
Other methods of dye addition are taught as exemplified in, for example, Diehl, et al., U.S. Pat. No. 4,948,717, wherein filter dyes are added as a solid dye dispersion. However, this method is not useful for sensitizing dyes or the addition of other solid materials to an emulsion. Handling of a solid dye with this technology typically generates airborne dust particles which may cause respiratory problems.
In light of these techniques, there is a pressing need in the art for a method of adding a wide array of solid materials to a photographic emulsion without the problems associated with solution techniques, microencapsulation techniques and solid particle additions.